Radio spectrum is typically allocated by Government organisations. Portions of the radio spectrum are licensed to users for particular use. For example AM and FM radio bands are licensed to different radio service providers. Unlicensed users of spectrum are generally tracked down and stopped from using licensed portions of the spectrum. Some areas of the spectrum are unlicensed and provide a free for all for any number of users. Examples of unlicensed spectrum include the spectrum in which baby monitors, garage door openers and cordless telephones operate; this is the 2.4 GHz ISM band.
One of the inefficiencies of licensing radio spectrum is that inefficient use is made of the licensed spectrum. With a spectrum license to a radio station, only that radio station can use its licensed portion of spectrum. It may be possible for other users to use the same spectrum without causing interference to the licensed party but under the current licensing terms, this is not allowed.
The commercial success of unlicensed spectrum is leading to developments in which many wireless users can operate in the same frequency band and share the spectrum. Some existing systems to allow this include dynamic frequency selection and transmit power control. These new concepts have lead to a new smart wireless system called cognitive radio. An important aspect of cognitive radio behaviour is the ability to reliably detect the present of interference from other systems for competing spectrum use. This is necessary both in the cognitive radio receiver to protect the integrity of the wanted signal and also on the cognitive radio transmitter, which must avoid producing interference on other systems. At the receiver, robust interference detection allows the system to identify which channels or bands should be avoided. Further, where the system in question is broadband, the interference is relatively narrow, it is possible for the system to co-exist in the same spectrum as the interfering system with minimal disruption to either system.
Interference can either be wideband or narrowband. Detection of narrowband interference is based on well established principles including maximum likelihood estimation, exploiting the inherent cyclostationarity of narrowband signals. However, as more wideband communication systems are developed and standardised, interference increasingly occurs between wideband systems. In this interference the spectral use of one system may fully or partially overlap the spectral use of another system. While some wideband modulations such as orthogonal frequency division multiplexing exhibit cyclostationarity, wideband systems are not inherently cyclostationary. Wideband interference detection must be based primarily on received power spectrum density. Moreover, where wideband interference occurs between heterogeneous systems, each cognitive radio receiver must either view the receiver signal of the others as being generated by an unknown stochastic process or else incorporate a full physical layer receiver for each interfering system. Some existing systems use an FFT to determine the power in each frequency bin. If the power is more than a threshold value, the detector determines that there is interference. This method is a simple interference temperature method. Another proposed system is to use a control channel common to all users of the spectrum. The control channel will provide information on the spectral usage of each user. However, such a system relies on different manufacturers all agreeing to the same protocols and frequency for the control channel. The control channel system avoids finding and suppressing interference. In the worse case, the system is completing blind to any interferers; the interferers are wideband with unknown frequency and unknown statistics.